Fluvial Erosion Erosion occurs in rivers because they have ENERGY Total energy of a river depends on: Weight of the water (i.e. a greater mass of water = more energy due to gravity) Height above base level (a river’s aim in life is to reach the sea, the higher it is the more potential energy it has) Steepness of channel (controls speed and therefore kinetic energy of the river) 1

Fluvial Erosion Corrasion (Abrasion) Attrition Solution Running water is one of the most effective erosion agents, breaking up the bedrock over which it flows and wearing down the land by chemically dissolving rocks or using sand, pebbles, and even boulders as cutting tools, as it grinds and hammers away at its bed and banks. As the velocity of the river increases, so to does the load it can carry and the rate at which it can erode. A river may erode by one of 4 processes: Corrasion (Abrasion) Attrition Solution Hydraulic Action 2

Fluvial Erosion – Corrasion (Abrasion) The rubbing or scouring of the bed and banks by sediment carried along by the river. This can vary from fine particles kept in suspension by turbulent flow to heavier boulders rolled along at times of bank-full flow. Major method by which river erodes both vertically and horizontally. Imagine sandpaper rubbing along the bed/banks wearing it away. Example Landform: potholes which are formed when turbulent eddies in the current swirl pebbles around to form potholes. These are hollows in river bed which trap pebbels. 3

Fluvial Erosion – Attrition Refers to reduction in the size of the sediment particles as they collide with each other, the bed and banks. Pieces of sediment become smaller and more rounded as they move downstream, so it is more common to find rounder, smaller fragments of rock downstream and coarser, more angular fragments upstream. Eventually attrition causes rock to turn into fine silt. Remember it by thinking about two rocks hitting each other. 4

Fluvial Erosion – Solution This process is independent of river discharge and velocity. Weak acids in the water dissolve minerals in the rock. Most active on rocks that contain carbonates such as limestone and chalk. 5

Fluvial Erosion – Hydraulic Action The sheer force of water eroding the bank and bed, in a number of ways: The turbulent water current hits river banks and pushes water into cracks. The air in cracks compressed, pressure increased and in time bank will collapse (also called Cavitation). The water simply picking up loose sediment by frictional drag of moving water. 6

Fluvial Erosion Rivers may erode horizontally and vertically, and often it happens at the same time. Vertical Erosion This is a characteristic of faster flowing rivers where there is enough sediment to down-cut, also as there is larger and more angular boulders being moved by the high velocity and there is a more angular bed-load, it creates a relatively quick lowering of the channel floor, generating steep-sided valleys. Horizontal Erosion (Lateral Erosion) When a river has a sizeable flood-plain it may meander across the valley, meaning lateral erosion will dominate. This happens particularly when the flood-plain is composed of alluvial (clay, silt or gravel) sediments, as hydraulic action can attack the outside of the meander bend, leading to undercutting and eventual collapse of river banks. 7

       Fluvial Erosion – Soil Type e.g. SAND is more easily eroded than CLAY This is because clay (and silt) particles are very loyal friends… they stick together in times of trouble! Sand particles are loners… they don’t care if their friends are being washed away….  I love you Heeellpp! I’m being eroded!!       I love you too! And? Who cares… 8

Fluvial Transport The 2 Cs Capacity Competence There’s 2 important words you need to know in relation to river transport. The 2 Cs Capacity Competence the total sediment load of a river at a particular point or location. Capacity increases to the 3rd power of its velocity. the size of the largest sediment particle that can be carried by that river at a particular time or location. Competence increases to the 6th power of its velocity 9

Fluvial Transportation The load is transported by 4 ways: Saltation: when pebbles, sand and gravel (bedload) are lifted up by current and bounced along the bed in a hopping motion. (ii) Traction: when largest boulders and cobbles (bedload) roll or slide along the bed. 10

Fluvial Transportation Suspension: very fine particles such as clay and silt (suspended load) are dislodged and carried by turbulence in a fast flowing river. Solution: water flowing within a river channel contains acids (e.g. carbonic acid from precipitation) dissolve the load such as limestone in running water and removed in solution. 11

Fluvial Transportation Once the river has eroded material from it’s bed and banks, or material has fallen or been washed into the river, it can be transported downstream in several ways. Sketch a river like the one below and add annotated diagrams of each type of fluvial transport. Use p. 14 of your textbook to help you. Define the terms bedload, suspended load and dissolved load. 25 12

Fluvial Deposition Deposition happens when a river is no longer competent or has lost the capacity to carry all of it’s load. Any reduction in river velocity will mean in material being deposited, starting with the coarsest sediment as this requires a lot of energy to remain in suspension. When does it occur? There is a sudden reduction in gradient (e.g. foot of a mountain range) The river enters a lake or the sea Discharge has been reduced following a period of little rainfall Where there is shallower water (e.g. The inside of a meander) There is a sudden increase in the volume of sediment available, such as at a confluence / where a landslide has occurred River overflows its bank so velocity outside channel is reduced. (resulting in floodplain) 13